(1) Field of the Invention
This invention relates to radiation detecting apparatus of the direct conversion type used in the medical, industrial, nuclear and other fields, and more particularly to a technique of improving the sensitivity for detecting radiation.
(2) Description of the Related Art
Radiation detecting apparatus employing semiconductor detectors include the indirect conversion type which first converts radiation (e.g. X rays) into light and then converts the light into electric signals by photoelectric conversion, and the direct conversion type which converts incident radiation directly into electric signals with a radiation sensitive semiconductor layer. The direct conversion type apparatus has electrodes formed on opposite surfaces of the radiation sensitive semiconductor layer. A predetermined voltage is applied to one of the electrodes (voltage application electrode). The other electrode (carrier collection electrode) collects carriers generated by incident radiation. The carriers are taken out as radiation detection signals, thereby enabling a detection of the radiation.
A conventional radiation detecting apparatus of the direct conversion type employs a single crystal semiconductor as a material for the semiconductor layer. The single crystal semiconductor is used since it has excellent carrier transport characteristics compared with an amorphous or polycrystal semiconductor having numerous localized levels and grain boundaries. The conventional radiation detecting apparatus, with the excellent carrier transport characteristics of the single crystal semiconductor, is used as a radiation energy detector or a radiation pulse counter.
Since only the carriers generated by incident radiation are taken out as signals, the conventional radiation detecting apparatus needs structures on both the opposite electrode sides for inhibiting carrier injection from the electrodes. Thus, the conventional apparatus has the following specific construction.
As shown in FIG. 1, the conventional apparatus may include a single crystal semiconductor layer 51 of high specific resistance, which is doped with impurities to make one side p-type and the other side n-type. Structures 52 and 53 are formed on the opposite sides of semiconductor layer 51 to inhibit injection of electrons e and holes h, respectively. Alternatively, as shown in FIG. 2, the conventional apparatus may include an n-type (or p-type) single crystal semiconductor layer 61 of relatively low specific resistance, which may be doped with an impurity to make one side p-type (or n-type) or may have a structure 62 in the form of a barrier metal electrode on the one side to inhibit injection of electrons e (or holes h). The other side is guarded against injection of holes h (or electrons e) by the conductivity of the crystal per se. In FIGS. 1 and 2, the left electrode acts as a bias voltage application electrode, and the right electrode as a carrier collection electrode.
However, the conventional radiation detecting apparatus has a drawback that it can hardly meet a demand for an enlarged detecting area. In each of the medical, industrial, nuclear and other fields today, there is a strong demand for a semiconductor type radiation detecting apparatus with a large area and high sensitivity to realize a high-speed, highly sensitive and compact radiation detecting system. Under current conditions, a 3-inch diameter is the limit for radiation detecting, single crystal semiconductor materials. It is difficult to realize a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
With an amorphous or polycrystal semiconductor, on the other hand, 1000 cm2 and larger sizes may be achieved by using a thin film making technique. However, sensitivity is insufficient because of the presence of localized levels and grain boundaries. Here again, it is difficult to realize a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
This invention has been made having regard to the state of the art noted above, and its object is to provide a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
The above object is fulfilled, according to a first aspect of this invention, by a radiation detecting apparatus for converting incident radiation into electric signals, comprising:
a radiation sensitive semiconductor layer for generating carriers in form of electron-hole pairs in response to the incident radiation;
a pair of electrodes formed on opposite surfaces of the semiconductor layer, respectively, one of the electrodes being a voltage application electrode to which a negative bias voltage is applied, the other electrode being a carrier collection electrode;
a charge storing capacitor connected to the carrier collection electrode for storing charges generated by movement of the carriers in the semiconductor layer;
a switching element connected to the capacitor, the switching element being turned off when the charges accumulate in the capacitor, and turned on when taking the charges out of the capacitor; and
charge-to-voltage converter means for converting the charges taken out of the capacitor through the switching element into voltage signals acting as radiation detection signals;
wherein the semiconductor layer is formed of one of an n-type amorphous semiconductor and an n-type polycrystal semiconductor, both being of high specific resistance, with a xcexcxcfx84 product of holes being larger than a xcexcxcfx84 product of electrons, the xcexcxcfx84 product being a product of mobility xcexc and mean life xcfx84 of the electrons and holes generated by the incident radiation; and
wherein the semiconductor layer has a structure formed on a voltage application electrode side thereof for inhibiting injection of electrons, and a structure formed on a carrier collection electrode side for permitting injection of holes.
The radiation detecting apparatus in the first aspect of the invention, as shown in FIG. 3, includes a radiation sensitive semiconductor layer 1A formed of an n-type amorphous or polycrystal semiconductor of high specific resistance, with the xcexcxcfx84 product of holes h larger than the xcexcxcfx84 product of electrons e among the carriers generated. When radiation is detected by this detecting apparatus, the holes h which are the minority carriers in the semiconductor layer 1A contribute to the radiation detection. A negative bias voltage xe2x88x92VA is applied from a bias voltage supply Ve to one electrode side of semiconductor layer 1A. Consequently, holes h are promptly injected from a hole injection permitting structure 3A formed on the carrier collection electrode side of semiconductor layer 1A, in a quantity corresponding to the quantity of the carriers generated by the incident radiation. The semiconductor layer 1A has a high specific resistance, whereby the injection of holes h is not inhibited by the conductivity of the semiconductor layer 1A per se. On the other hand, an injection of electrons e is inhibited by an electron injection inhibiting structure 2A formed on the voltage application electrode side of semiconductor layer 1A.
In the prior art, the injection of both holes h and electrons e is inhibited. The first aspect of the invention inhibits only the injection of electrons e which are the majority carriers not contributing to the improvement in sensitivity. Only the injection of holes h which contribute to the improvement in sensitivity is permitted to improve the sensitivity by a degree corresponding to the increase in the holes h. The holes h injected are the minority carriers in the semiconductor layer 1A, and thus an increase in dark current is suppressed. Of course, the detection area may be enlarged since the semiconductor layer 1A is formed of an amorphous or polycrystal semiconductor material.
Charges generated by movement of holes h accumulate in a charge storing capacitor Ca while a switching element 4 is turned off. When the switching element 4 is turned on, the charges stored are read from a charge-to-voltage converter 5 as voltage signals acting as radiation detection signals. Thus, a detecting operation is not interrupted, with holes h accumulating in the capacitor Ca, even during a non-signal reading period, which provides an advantage in terms of sensitivity. Further, in the case of a multichannel construction, the charge-to-voltage converter 5 for reading detection signals may be arranged switchable for connection to a plurality of capacitors, thereby reducing the number of charge-to-voltage converters 5 required.
Next, the detecting sensitivity of the radiation detecting apparatus in the first aspect of the invention will be described in terms of quantity.
Current i flowing when an electric field E is applied and radiation (X rays) is emitted to a semiconductor layer 1 having a sectional area S and a thickness corresponding to a distance d between the electrodes, is expressed by i=xcex94i+iD (where xcex94i is a signal current portion, and iD is a dark current, portion). The signal current portion xcex94i is expressed by the following equation (1):
xcex94i=qxcex94nxc2x7xcexcES=q(xcex94nexcexce+xcex94nhxcexch)ESxe2x80x83xe2x80x83(1)
where xcex94n is an increase in carriers, xcexcE is a moving speed of the carriers, xcex94ne is an increase in electrons, xcex94nh is an increase in holes, and q is a quantum of electricity.
On the other hand, where the quantity of carriers generated per unit volume and unit time is g (cmxe2x88x923 secxe2x88x921), since irradiation is assumed to be steady (dg/dt=0), a change with the passage of time of xcex94n is expressed by dxcex94n(t)/dt=gxe2x88x92xcex94n(t)/xcfx84 (where xcfx84 is a mean life of the carriers). This equation is solved to obtain xcex94n(t)=gxcfx84[1xe2x88x92exp(xe2x88x92t/xcfx84)]. Where t greater than  greater than xcfx84, xcex94n(t)=gxcfx84 and the following equations (2) and (3) are obtained:
xcex94ne=gexcfx84exe2x80x83xe2x80x83(2)
xcex94nh=ghxcfx84hxe2x80x83xe2x80x83(3)
Since ge=gh=g, the above equations are substituted into equation (1) to obtain equation (4):
xcex94i=qgES(xcexcexcfx84e+xcexchxcfx84h)=qgdS(xcexcexcfx84eE/d)+(xcexchxcfx84hE/d)xe2x80x83xe2x80x83(4)
Further, since the injection of electrons e is inhibited here, equation (4) becomes equation (5).
xcex94i=qgdS(xcexchxcfx84hE)/dxe2x80x83xe2x80x83(5)
In this case, since xcexce xcfx84e less than  less than xcexch xcfx84h, the value of xcex94i hardly changes.
In the conventional radiation detecting apparatus, by contrast, since ge and gh in the equations (2) and (3) are replaceable with Hecht""s equations (6) and (7) below, equation (4) above becomes equation (8):
ge=ge[1xe2x88x92exp[xe2x88x92(dxe2x88x92r)/xcexcexcfx84eE]]xe2x80x83xe2x80x83(6)
xe2x80x83gh=gh[1xe2x88x92exp[xe2x88x92r/xcexchxcfx84hE]]xe2x80x83xe2x80x83(7)
xcex94i=qdS{ge[1xe2x88x92exp[xe2x88x92(dxe2x88x92r)/xcexcexcfx84eE]](xcexcexcfx84eE/d)+gh[1xe2x88x92exp[xe2x88x92r/xcexchxcfx84hE]](xcexchxcfx84hE/d)}xe2x80x83xe2x80x83(8)
Further, the expression in { } of equation (8) approaches [ge (dxe2x88x92r)+gh r]/d as E increases. This value is equal to g, and therefore equation (8) finally becomes equation (9).
xcex94iMAX=qgdSxe2x80x83xe2x80x83(9)
Equation (9) shows that, in the conventional (non-injection type) radiation detecting apparatus, signals do not depend on electric field E, but depend on g or carriers generated.
A comparison between equation (5) for the first aspect of this invention and equation (9) for the prior art shows that, in the first aspect of the invention, Ai increases by (xcexch xcfx84h E)/d times. The rate of increase is proportional to electric field E, and electric field E=bias voltage/distance between the electrodes. Consequently, the improvement in sensitivity increases with the bias voltage.
In a second aspect of the invention, there is provided a radiation detecting apparatus for converting incident radiation into electric signals, comprising:
a radiation sensitive semiconductor layer for generating carriers in form of electron-hole pairs in response to the incident radiation;
a pair of electrodes formed on opposite surfaces of the semiconductor layer, respectively, one of the electrodes being a voltage application electrode to which a positive bias voltage is applied, the other electrode being a carrier collection electrode;
a charge storing capacitor connected to the carrier collection electrode for storing charges generated by movement of the carriers in the semiconductor layer;
a switching element connected to the capacitor, the switching element being turned off when the charges accumulate in the capacitor, and turned on when taking the charges out of the capacitor; and
a charge-to-voltage converter for converting the charges taken out of the capacitor through the switching element into voltage signals acting as radiation detection signals;
wherein the semiconductor layer is formed of one of an n-type amorphous semiconductor and an n-type polycrystal semiconductor, both being of high specific resistance, with a xcexcxcfx84 product of holes being larger than a xcexcxcfx84 product of electrons, the xcexcxcfx84 product being a product of mobility xcexc and mean life xcfx84 of the electrons and holes generated by the incident radiation; and
wherein the semiconductor layer has a structure formed on a voltage application electrode side thereof for permitting injection of holes, and a structure formed on a carrier collection electrode side for inhibiting injection of electrons.
With the radiation detecting apparatus in the second aspect of the invention, as shown in FIG. 4, a positive bias voltage +VA is applied from the bias voltage supply Ve to one electrode side of a semiconductor layer 1B. The semiconductor layer 1B has a hole injection permitting structure 2B formed on a voltage application electrode side thereof, and an electron injection inhibiting structure 3B on a carrier collection electrode side. This apparatus has the same functions as the apparatus in the first aspect of the invention except that the direction for injecting holes h is reversed. Thus, the other details will not be discussed.
In a third aspect of the invention, there is provided a radiation detecting apparatus for converting incident radiation into electric signals, comprising:
a radiation sensitive semiconductor layer for generating carriers in form of electron-hole pairs in response to the incident radiation;
a pair of electrodes formed on opposite surfaces of the semiconductor layer, respectively, one of the electrodes being a voltage application electrode to which a negative bias voltage is applied, the other electrode being a carrier collection electrode;
a charge storing capacitor connected to the carrier collection electrode for storing charges generated by movement of the carriers in the semiconductor layer;
a switching element connected to the capacitor, the switching element being turned off when the charges accumulate in the capacitor, and turned on when taking the charges out of the capacitor; and
a charge-to-voltage converter for converting the charges taken out of the capacitor through the switching element into voltage signals acting as radiation detection signals;
wherein the semiconductor layer is formed of one of a p-type amorphous semiconductor and a p-type polycrystal semiconductor, both being of high specific resistance, with a xcexcxcfx84 product of electrons being larger than a xcexcxcfx84 product of holes, the xcexcxcfx84 product being a product of mobility xcexc and mean life xcfx84 of the electrons and holes generated by the incident radiation; and
wherein the semiconductor layer has a structure formed on a voltage application electrode side thereof for permitting injection of electrons, and a structure formed on a carrier collection electrode side for inhibiting injection of holes.
With the radiation detecting apparatus in the third aspect of the invention, as shown in FIG. 5, a radiation sensitive semiconductor layer 1C is formed of a p-type amorphous or polycrystal semiconductor of high specific resistance, with the xcexcxcfx84 product of electrons e larger than the xcexcxcfx84 product of holes h among the carriers generated. The electrons e which are the minority carriers in the semiconductor layer 1C contribute to the radiation detection. A negative bias voltage xe2x88x92VA is applied from the bias voltage supply Ve to one electrode side of semiconductor layer 1C. Electrons e are promptly injected from an electron injection permitting structure 2C formed on the electron application electrode side of semiconductor layer 1C, in a quantity corresponding to the quantity of the carriers generated by incident radiation. On the other hand, the semiconductor layer 1C has a hole injection inhibiting structure 3C formed on the voltage application electrode side thereof Thus, holes h are not injected, but only electrons e are injected.
Since the injection of holes h is inhibited here, the foregoing equation (5) becomes equation (10) below:
xcex94i=qgdS(xcexcexcfx84eE)/dxe2x80x83xe2x80x83(10)
Since xcexch xcfx84h less than  less than xcexce xcfx84e, the value of xcex94i, of course, hardly changes. Sensitivity is improved since xcex94i increases by (xcexce xcfx84e E)/d times.
This apparatus has the same functions as the apparatus in the first aspect of the invention in the other details. Thus, the other details will not be discussed.
In a fourth aspect of the invention, there is provided a radiation detecting apparatus for converting incident radiation into electric signals, comprising:
a radiation sensitive semiconductor layer for generating carriers in form of electron-hole pairs in response to the incident radiation;
a pair of electrodes formed on opposite surfaces of the semiconductor layer, respectively, one of the electrodes being a voltage application electrode to which a positive bias voltage is applied, the other electrode being a carrier collection electrode;
a charge storing capacitor connected to the carrier collection electrode for storing charges generated by movement of the carriers in the semiconductor layer;
a switching element connected to the capacitor, the switching element being turned off when the charges accumulate in the capacitor, and turned on when taking the charges out of the capacitor; and
a charge-to-voltage converter for converting the charges taken out of the capacitor through the switching element into voltage signals acting as radiation detection signals;
wherein the semiconductor layer is formed of one of a p-type amorphous semiconductor and a p-type polycrystal semiconductor, both being of high specific resistance, with a xcexcxcfx84 product of electrons being larger than a xcexcxcfx84 product of holes, the xcexcxcfx84 product being a product of mobility xcexc and mean life xcfx84 of the electrons and holes generated by the incident radiation; and
wherein the semiconductor layer has a structure formed on a voltage application electrode side thereof for inhibiting injection of holes, and a structure formed on a carrier collection electrode side for permitting injection of electrons.
With the radiation detecting apparatus in the fourth aspect of the invention, as shown in FIG. 6, a positive bias voltage +VA is applied from the bias voltage supply Ve to one electrode side of a semiconductor layer 1D. The semiconductor layer 1D has an electron injection permitting structure 3D formed on a carrier collection electrode side thereof, and a hole injection inhibiting structure 2D on a voltage application electrode side. This apparatus has the same functions as the apparatus in the third aspect of the invention except that the direction for injecting electrons e is reversed. Thus, the other details will not be discussed.
In the radiation detecting apparatus in the first to fourth aspects of the invention, preferably, the bias voltage applied to the voltage application electrode has a value within a range where an absolute value |VA| of the bias voltage is |VA| greater than d2/xcexcxcfx84L, where d is a distance between the electrodes, and xcexcxcfx84L is the larger of the xcexcxcfx84 products of the carriers.
With the bias voltage set as above, xcex94i in equation (5) or (10) is larger than xcex94i in equation (9), thereby reliably performing a radiation detecting operation with high sensitivity.